1. Field of the Invention
This invention relates to a laser. This invention particularly relates to a laser, wherein an etalon is located in a laser resonator in order to bring the oscillation mode to a single longitudinal mode. This invention also relates to an etalon for use in the laser and a method for making the etalon.
2. Description of the Prior Art
Laser diode pumped solid lasers have been proposed in, for example, U.S. Pat. No. 4,656,635. The proposed laser diode pumped solid lasers comprise a solid laser rod, which has been doped with a rare earth metal, such as neodymium (Nd). The solid laser rod is pumped by a semiconductor laser (a laser diode).
As described in, for example, Optics Letters, Vol. 18, 1993, p. 420, in the laser diode pumped solid laser of this type, an etalon is often located in the laser resonator in order to effect wavelength selection and to bring the oscillation mode to the single longitudinal mode such that the occurrence of noise due to mode competition can be restricted.
However, with the conventional laser in which an etalon is used in order to bring the oscillation mode to the single longitudinal mode, the threshold value of the pumping power for the oscillation of the laser becomes markedly large. As a result, the problems often occur in that, for example, the output power of the laser becomes low. The increase in the threshold value occurs because the etalon longitudinal mode intervals are set to be sufficiently large with respect to the wavelength width of the laser beam capable of being radiated from the laser medium. Such setting of the etalon longitudinal mode intervals is carried out in order to prevent the problems from occurring in that a plurality of etalon longitudinal modes occur in the wavelength width of the laser beam capable of being radiated from the laser medium and the oscillation mode cannot be brought to the single longitudinal mode.
Specifically, in general, in such a structure, if the thickness of the etalon is not controlled to the submicron order, the etalon longitudinal mode and the gain peak wavelength of the laser medium will not coincide with each other. In such cases, the gain is not utilized efficiently, and therefore the oscillation threshold value becomes large.
It is very difficult to control the thickness of the etalon to the submicron order by a polishing process. Even if such control is possible, a very high cost will be required to carry out such control. Therefore, an attempt has heretofore been made to adjust the optical path length of the etalon by adjusting the inclination of the etalon, which is inserted into the resonator, with respect to the resonator axis such that the etalon longitudinal mode and the gain peak wavelength of the laser medium may coincide with each other.
However, if the inclination of the etalon becomes very large, etalon insertion loss will become large. Therefore, the threshold value of the pumping power for the oscillation of the laser will become large, and the problems will occur in that, for example, the output power of the laser becomes low. If the inclination of the etalon is very small, the required level of the output power of the laser can be obtained, but the problems will occur in that, for example, the single longitudinal mode characteristics become bad. Also, in cases where laser apparatuses are to be massproduced, it is necessary to carry out the adjustment of the angle of the etalon for each of the laser apparatuses. Therefore, the problems occur in that the productivity becomes low, and the specifications, such as the threshold value of the pumping power for the oscillation of the laser, and the output power of the laser, vary for different laser apparatuses. Further, in cases where it is necessary for the etalon to be inclined at a large angle for the purposes of the adjustment of the optical path length, the problems occur in that the profile of the radiated beam becomes distorted due to such inclination of the etalon.